A sensor network system including a plurality of sensor nodes is known. Each of the sensor nodes detects a physical quantity (e.g., temperature, humidity, electricity consumption, or the like). Each of the sensor nodes transmits a packet containing sensor data representing the detected physical quantity to a specific node (a sink node) via the other sensor node. That is to say, the sensor nodes configure a wireless multihop network.
This type of sensor network system usually forms (constructs) a network topology having a tree structure using each of the sensor nodes as a node. A route node in the tree structure is the abovementioned sink node.
Such a network topology is usually constructed by using a routing protocol suitable for a wireless multihop network. As a routing protocol suitable for a wireless multihop network, a proactive routing protocol and a reactive routing protocol are known.
A typical example of a proactive routing protocol is the OLSR (Optimized Link State Routing) protocol. The OLSR protocol is a protocol defined by IETF (Internet Engineering Task Force) (see Non-Patent Document 1).
A typical example of a reactive routing protocol is the AODV (Ad hoc On-Demand Distance Vector) protocol, or the DSR (Dynamic Source Routing) protocol. The AODV protocol and the DSR protocol are protocols defined by IETF (see Non-Patent Document 2 and Non-Patent Document 3)
The routing protocol calculates a metric value representing cost (path cost) required for communication to a sink node, based on the intensity of radio signals received from another sensor node and/or a hop count from a sensor node to the sink node. Moreover, the routing protocol decides a parent node for each sensor node based on the calculated metric value, thereby constructing a network topology.
Further, as one of this type of sensor network systems, a sensor network system described in Patent Document 1 is configured to make the number of child nodes owned by each sensor node close to the average value.    Patent Document 1: Japanese Unexamined Patent Application Publication No. 2010-166150    Non-Patent Document 1: IETF MANET RFC3626: Optimized Link State Routing Protocol (OLSR)    Non-Patent Document 2: IETF MANET RFC3561: Ad hoc On-Demand Distance Vector (AODV) Routing    Non-Patent Document 3: IETF MANET RFC4728: The Dynamic Source Routing Protocol (DSR)
It is preferred that a sensor node is configured to, when receiving packets (pre-aggregation packets) containing sensor data from respective child nodes, generate a packet (a post-aggregation packet) containing the sensor data contained in each of the received packets and transmit the generated packet to a parent node. This enables reduction of the number of packets transmitted from the sensor node to the parent node.
On the other hand, the size of data that one packet can contain is previously determined. Therefore, in a case where the number of the child nodes owned by the sensor node is equal to a maximum containing number, which is the maximum number of sensor data that one packet can contain, the sensor node transmits one post-aggregation packet to the parent node. Moreover, for example, in a case where the number of the child nodes owned by the sensor node is 1 larger than the maximum containing number, the sensor node transmits two post-aggregation nodes to the parent node.
The abovementioned sensor network system constructs a network topology regardless of the maximum containing number. Therefore, there is a fear that the number of packets transmitted by the sensor node to the parent node becomes larger than necessary. That is to say, in the abovementioned sensor network system, communication load possibly becomes excessively large.